Method of producing fluoride-free phosphoric acid

ABSTRACT

Substantially fluoride-free phosphoric acid is produced by the acidulation of phosphate rock with sulfuric acid wherein the acidulation reaction is conducted in the presence of added potassium as K 2  O bearing compounds, e.g. KHSO 4 , K 2  SO 4 , KH 2  PO 4  or KOH, and additional silica, to precipitate fluorides and silica as potassium silicofluoride and then removed on separation of the gypsum formed. Optionally, additional silica may be added to the phosphoric acid solution recovered after precipitation and removal of the gypsum, to precipitate additional fluoride as potassium silicofluoride to further purify the phosphoric acid. In a further embodiment sufficient potassium may be added along with additional silica in the acidulation step to produce potassium dihydrogen phosphate and phosphoric acid as additional products. The phosphoric acid and potassium dihydrogen phosphate are recovered substantially free from fluoride contamination and the reaction is carried out in the substantial absence of fluorine evolution into the atmosphere. Also disclosed is a process for conducting a conventional phosphoric acid facility to produce phosphoric acid and gypsum wherein defluorination of the phosphoric acid recovered is achieved by silicon dioxide and K 2  O addition and the fluorides are removed from the system, the process including steps for production of pure phosphoric acid and potassium dihydrogen phosphate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the production of fluoride-free phosphoricacid, and optionally substantially fluoride-free potassium dihydrogenphosphate, by the acidulation of phosphate rock with sulfuric acid, andmore particularly to a process comprising the acidulation of phosphaterock with sulfuric acid wherein the reaction is conducted in thepresence of a controlled amount of potassium ion as K₂ O and acontrolled amount of silica as SiO₂.

2. Description of the Prior Art

Phosphoric acid plants are currently operated utilizing a basic and wellknown process for the acidulation of phosphate rock which comprisesreaction of the rock with sulfuric acid to form phosphoric acid withsubsequent reaction of the phosphoric acid with ammonia to produceammonium phosphates. The phosphoric acid formed in this process iscalled wet process phosphoric acid. In this reaction, a by-product isgypsum having the chemical formula CaSO₄.2H₂ O. Essentially, allphosphate rock contains some fluorine normally in the 3.0-4.0 percentrange and the acidulation reaction usually generates gaseous fluorideswhich in prior years was usually evolved into the atmosphere or trappedwith water scrubbing apparatus.

In recent years, air and water pollution law regulations have becomemore stringent and are now being enforced vigorously. Therefore,operators of phosphoric acid plants have had many pollution problemswith fluorine emission in the atmosphere and with the by-product gypsumfrom these phosphoric acid plants. An important problem in the operationof these wet process phosphoric acid plants has been in the expensivemethods necessary for handling the large amounts of fluorine compoundsliberated in the gaseous and aqueous effluents from such plants. In somephosphate complexes from 10,000 to 30,000 tons per year of fluorinecompounds may be liberated by various methods and it is estimated thatin a typical wet process phosphoric acid plant, a portion of thefluorides are evolved in the atmosphere in gaseous form such as hydrogenfluoride and silicon tetrafluoride which can destroy vegetation andaffect other facilities in close proximity to the plant if they are notscrubbed out, and such scrubbing systems are not always effective. Asecond portion of the fluorine is found in gypsum ponds and is subjectto reaching into groundwater and streams. Still another portion of thefluorine remains with the final products and when such final productsare used as fertilizers they may introduce fluorine into the soil. Onlyin recent years have studies been made on the effects of fluoridescontained in final products and indications seem clear that they mayhave a deleterious effect on the long range producing ability of thesoil, see for Example Kudzin et al., Chemical Abstracts, 73, 870534(1970).

There is a great deal of literature and patent art related to attemptsto remove the fluoride values from fluorine-containing phosphate rock inthe operation of a phosphoric acid plant including methods forsuppressing the evolution of fluoride values in the operation of aprocess and/or attempting to scrub the fluorine from effluent gases andwaste water. Two such methods are described in U.S. Pat. Nos. 2,954,275and 2,976,141 to Carothers et al. which use sodium or potassiumcompounds to suppress the fluorides so that they are concentrated in thegypsum cake. These patents indicate that this is achieved by adding asuppressing amount of an alkali metal salt to the acidulation reaction.However, these processes were conducted in the presence of sulfuric acidin the acidulation reactor and the process had incomplete control onfluoride decomposition and evolution during acidulation.

Other prior art has been noted which attempts to overcome the problem offluorine evolution and a reduction in the amount of fluorine containedin final products. A reference of this type is British Pat. No. 735,086(1955), which discloses the acidulation of phosphate rock by a two stepprocedure using a strong mineral acid such as nitric acid orhydrochloric acid. According to this patent, an initial low temperatureacidulation at 20°-50° C. is carried out with the addition of an alkali,for example, ammonia or lime, as a precipitating agent in a quantitysufficient to precipitate substantially the whole of the fluorine andother impurities but insufficient to precipitate a substantial amount ofcalcium phosphates.

In a similar process in U.S. Pat. No. 3,431,096 to Hill et al., aprocess is disclosed for reducing evolution of fluorine values information of triple superphosphate fertilizer by reaction of phosphaterock and phosphoric acid wherein ammonia or urea is added to suppressthe fluorine evolution. However, in this patent, there is no provisionfor removal of the fluorine values from the product and therefore evenif the fluorine evolution is prevented, the fluorine values will beretained in the resulting product and therefore distributed to the soilwhen it is used as a fertilizer.

In a series of patents issuing from the mid 1940's to early 1960's thereare disclosed processes for the defluorination of phosphate rock and theproduction of defluorinated calcium phosphates. In these patents, U.S.Pat. Nos. 2,337,498; 2,442,969; 2,893,834; and 2,997,367, thedefluorination reaction is carried out by subjecting a mixture ofphosphate rock, phosphoric acid and an alkali metal material tocalcination, that is by reaction at temperatures as high as 1000° C. to2200° C. Obviously, under these conditions the fluorine is going to berapidly evolved or if not evolved, certainly will remain in the finalproduct, said to be an animal feed.

Two additional patents of pertinence to processes of this type are U.S.Pat. Nos. 2,567,227 and 2,728,635 to Miller which disclose theacidulation of phosphate rock with phosphoric acid to form monocalciumphosphate, cooling to solidify the monocalcium phosphate and thenconverting it to dicalcium phosphate by hydrolysis. In the earlierpatent, it is indicated that the fluorine in the rock is vaporized inthe system, circulates throughout the system and/or leaves the systemwith the calcium phosphate. The later patent indicates that the processof U.S. Pat. No. 2,567,227 provided a final calcium phosphate producthaving a fluorine content too high to be of animal feed grade. Thesolution to this problem in the later patent was the addition of somedilute sulfuric acid in the acidulation step which would of course leadto additional fluorine evolution during the first step.

There are also patents known in the art which indicate that it is knownto acidulate phosphate rock with phosphoric acid and to then recoversolid monocalcium phosphate by cooling of the resulting solution andrecovering the monocalcium phosphate. Processes of this type aredisclosed for example in U.S. Pat. Nos. 3,494,735 and 3,645,676. Inaddition, U.S. Pat. Nos. 3,619,136 and 3,792,151 to Case disclose thereaction of phosphate rock with recycle phosphoric acid at temperaturesof about 125°-180° F. (52° C. to 83° C.) to form a solution ofmonocalcium phosphate, reacting the latter solution with sulfuric acidto produce phosphoric acid and calcium sulfate, precipitating thecalcium sulfate, and recycling a portion of the phosphoric acid to thephosphate rock reaction. These patents point out that under theconditions recited, fluorides are not evolved but remain primarilyunreacted and may be found with insoluble materials although a portionremains in the phosphoric acid solution product. It is also known toreact phosphate rock or a solubilized form with sulfuric acid and KHSO₄in combination with other steps and this reaction is described in U.S.Pat. Nos. 3,697,246 and 3,718,253.

In copending application Ser. No 608,973, filed Aug. 29, 1975, of one ofus, there is disclosed a process for the acidulation of phosphate rockand production of substantial pure alkali metal phosphates, calciumphosphates and phosphoric acid which comprises primarily the steps ofacidulating phosphate rock with a phosphoric acid solution containingsufficient alkali metal values to provide potassium ions in the systemand thus form an insoluble precipitate comprising a mixture ofimpurities, silicas and fluorides from which the fluorides can berecovered in usable form. It is a feature of this disclosure that thefluorides are not evolved into the atmosphere but are primarilyrecovered in the insoluble precipitate removed prior to gypsumprecipitation. In addition, in prior U.S. Pat. No. 3,840,639 there isdisclosed a process for the acidulation of phosphate rock by reaction ofthe rock with phosphoric acid in the presence of potassium ion.

Of other pertinent art in this area, U.S. Pat. No. 2,114,600 to Larsondiscloses the reaction of phosphate rock with nitric acid in order toform dicalcium phosphate and calcium nitrate. The patentee points outthat in this system difficulties are experienced because the fluorinepresent in phosphate rock is precipitated as a very slimy calciumfluoride extremely difficult to separate from the nitrate solution andto overcome this problem, the patentee suggests that the phosphate rockbe dissolved in the nitric acid in the presence of a fairly large amountof silica while at the same time adding an alkali salt such as alkalinitrate or chloride so that the fluorine reacts with the silica andalkali salt to provide an insoluble well crystalized alkalisilicofluoride which can easily be separated from the solution nitratesand phosphoric acid. The patent, however, is limited to the addition oflarge excesses over the theoretical amount of alkali salt.

A second patent in this area is U.S. Pat. No. 2,865,709 to Horn et alwhich relates to the production of insoluble silicofluorides whereinphosphate rock is mixed with sulfuric acid and the gas evolved from thesystem containing silicon tetrafluoride is absorbed in water to form awaste liquor solution containing fluosilicic acid. When this solution ismixed with a chloride such as potassium chloride, insolublefluosilicates are formed but the silica fluoride develops into a highlydispersed state as an unfilterable gel. This patentee proposes toovercome this problem by adjusting the fluorine to silica mol ratio inthe waste liquor so as to have a mol ratio of fluorine to silica ofabout 4.4:1 before reaction with potassium chloride.

The present invention provides an acidulation system which substantiallyeliminates the problem of fluorine evolution in the acidulation offluorine-containing phosphate rock with phosphoric acid and alsoprovides systems wherein substantially pure phosphoric acid isrecovered, and the fluoride contained in phosphate rock and phosphoricacid is precipitated as potassium silicofluoride. Therefore, the presentinvention provides a unique combination of steps and advantages notappreciated heretofore in the prior art as none of these priorreferences disclose the unique combination of steps and results of thisinvention.

SUMMARY OF THE INVENTION

It is accordingly one object of the invention to produce relatively purephosphoric acid and relatively pure alkali metal phosphates which areessentially free of fluorides.

A further object of the invention is to provide a procedure for theacidulation of phosphate rock with sulfuric acid wherein the fluoridesare caused to be precipitated and primarily recovered in combinationwith the gypsum.

A still further object of the invention is to provide a system for theacidulation of phosphate rock with sulfuric acid wherein the reaction iscarried out in the presence of potassium ion and silica.

A still further object of the invention is to provide a method for theacidulation of phosphate rock with sulfuric acid wherein the reaction isconducted in the presence of controlled amounts of potassium ion andsilica for the recovery of highly pure phosphoric acid and alternativelythe preparation of substantially pure potassium dihydrogen phosphate andphosphoric acid.

A still further object of the present invention is to provide a methodfor conducting the acidulation of phosphate rock with sulfuric acid withhydrogen fluoride evolution wherein the wet process phosphoric acidrecovered is subsequently subjected to defluorination to provide purefeed grade phosphoric acid and optionally, potassium dihydrogenphosphate product.

Other objects and advantages of the present invention will becomeapparent as the description thereof proceeds.

In satisfaction of the foregoing objects and advantages there isprovided by this invention a method for the acidulation of phosphaterock for the production of phosphoric acid which comprises contactingphosphate rock with a sufficient amount of sulfuric acid to effectacidulation wherein the reaction is conducted in the presence ofsufficient potassium ion that the resulting phosphoric acid filtratecontains about 0.5 to 1.0 weight percent K₂ O, and in the presence of acontrolled amount of silica in the acidulation system, conducting theacidulation until the precipitation of gypsum is complete, removing thegypsum containing therein fluoride as potassium silicofluoride andrecovering a solution of phosphoric acid. Also provided are optionalembodiments including the presence of a sufficient amount of potassiumion in the acidulation stage to produce potassium dihydrogen phosphateand phosphoric acid as well as a continuous system wherein the amount ofpotassium ion and silica required for the acidulation reaction areprovided by recycle of a stream containing K₂ O and SiO₂ with phosphoricacid. A further optional embodiment is the addition of further silicondioxide to the phosphoric acid solution to cause the precpitation of anyremaining fluoride in the phosphoric acid as potassium silicofluoride toprovide a feed grade phosphoric acid.

Also provided by this invention in a further optional embodiment is aprocess for conducting a conventional phosphoric acid facility involvingthe reaction of phosphate rock and sulfuric acid to form gypsum andphosphoric acid wherein the hydrogen fluorides are evolved and trappedin a scrubbing system, the gypsum precipitate is removed, and theresulting wet process phosphoric acid is subjected to defluorination byreaction with a silicon dioxide-containing material and K₂ O-containingmaterial in order to precipitate fluorides remaining in the phosphoricacid as potassium silicofluoride, removing the potassium silicofluorideand any additional gypsum from the phosphoric acid and treating forrecovery of the fluorides by reaction with calcium hydroxide, recoveringat least a portion of the phosphoric acid as substantially purephosphoric acid and reacting the remainder of the phosphoric acid withphosphate rock and potassium hydrogen sulfate or potassium sulfate toproduce potassium dihydrogen phosphate and gypsum. In one embodiment ofthis aspect, the potassium dihydrogen phosphate may be recovered, andthe remaining mixture of phosphoric acid, potassium dihydrogenphosphate, gypsum and any remaining fluorides recycled to thedefluorination step. In a separate and preferred embodiment, theprecipitated gypsum and fluorides as K₂ SiF₆ are removed to provide asolution having a certain P₂ O₅ /K₂ O content, a portion of the lattersolution is removed and the remainder is recycled to the defluorinationstep.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the drawing accompanying this applicationwherein:

FIG. 1 represents a schematic outline of the main embodiment of thepresent invention;

FIG. 2 represents a schematic outline of a continuous method includingan optional embodiment for conducting the process of this invention; and

FIG. 3 represents a schematic outline of a continuous method forconducting a conventional wet process phosphoric acid acidulationincluding embodiments for the recovery of defluorinated phosphoric acidand potassium dihydrogen phosphate products.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As indicated above, in a main embodiment, this invention is concernedwith a method for conducting the acidulation of phosphate rock withsulfuric acid or mixtures of sulfuric acid and phosphoric acid, whereinthe fluoride present in the rock is substantially prevented fromevolving during the reaction and thus contaminating the atmosphere, andalso is prevented from any significant presence in the final products.According to the present invention, it has been discovered that in orderto conduct the acidulation of phosphate rock with sulfuric acid in thesubstantial absence of fluorine evolution, and in the substantialabsence of fluoride contamination of final products, that it isnecessary to add controlled amounts of potassium ion as well ascontrolled amounts of silica as silicon dioxide in order to insurecomplete reaction of the fluoride contained in the rock with thepotassium and silicon and also to insure that the resulting product willprecipitate during the reaction and can thus be removed with the gypsumbyproduct. The present invention provides a procedure for conductingthis reaction and also provides procedures including continuousreactions whereby the acidulation reaction may be conducted utilizing arecycle stream to provide the necessary additional potassium and silicarequirements. Further, the continuous reaction also provides goodconversions to provide substantially pure products such as phosphoricacid and potassium dihydrogen phosphate.

In accordance with a main feature of the present invention, sulfuricacid and phosphate rock are reacted at temperatures ranging from 50°-90°C. with sufficient sulfuric acid being present to theoreticallycompletely acidulate the phosphate rock. The reaction is conducted at atemperature of about 50°-90° C., preferably 60° C. to 75° C. for a timesufficient to achieve substantially complete acidulation and adequategypsum crystal growth. It has been found that a reaction period of aboutthree to twelve hours is sufficient.

In order to suppress fluorine evolution and effect precipitation of thefluorine as potassium silicofluoride in conducting this process, thereis maintained in the acidulation reactor an amount of K₂ O as potassiumion in order to supply the amount necessary to combine with theavailable fluoride. Generally, it is preferred in this invention thatthe amount of potassium ion present in the mixture should be asufficient amount over and above the amount of potassium theoreticallynecessary to combine with the fluorine contained in the phosphte rock toprovide a K₂ O concentration of about 0.5 to 1.0 weight percent in theresulting phosphoric acid filtrate. The potassium ion may be provided asK₂ O by the addition to the mixture of KOH, KH₂ PO₄, KHSO₄, K₂ SO₄,mixtures thereof, and the like. Recycle of K₂ O as KH₂ PO₄ is especiallypreferred.

In this initial reaction the sulfuric acid acts on the rock to formphosphoric acid and gypsum, that is CaSO₄.2H₂ O. The 3-4% of fluorinepresent in the rock under ordinary circumstances would be evolvedbecause of the temperatures under which the reaction is conducted.However, with the addition of the indicated amount of potassium ion asK₂ O in the mixture and the silica already present in the rock, asubstantial portion of the fluorine will combine with the potassium andthe silica to form potassium silicofluoride. However, it has now beendiscovered that when this reaction was carried out previously, there wasnot sufficient silica present to combine with all the fluorine present.Therefore, it is a further feature of the present invention to add anadditional amount of silica in the form of silicon dioxide in order toprovide sufficient silica for all of the fluorine to combine withpotassium and the silica to form potassium silicofluoride, a compoundwhich will precipitate with the gypsum under the conditions of thereaction. The amount of additional silica added should range from about0.003 to 0.015 parts of SiO₂ to 1 part of P₂ O₅ in the resultingphosphoric acid filtrate. Therefore, as a result of this reaction, therewill be formed a solution of phosphoric acid with a precipitate ofgypsum in admixture with the potassium silicofluoride formed.

The mixture of slurry resulting from the initial reaction is thenseparated by use of a filter or other separation device, to provide asolution of phosphoric acid which will contain any excess potassium ionand a solids portion or filter cake comprising the mixture of gypsum andpotassium silicofluoride. The potassium silicofluoride may be separatedby extraction from the gypsum by the extraction methods disclosed in ourcopending application Ser. No. 696,289, filed of even date herewith.

In a further feature of the invention the phosphoric acid solution maybe treated with an additional amount of silica as silicon dioxide in anamount of about 0.1 to 0.5 percent of the total phosphoric acid weight,which will combine with any residual fluorine and the excess potassiumto precipitate additional potassium silicofluoride from the phosphoricacid. On removal of the K₂ SiF₆, the resulting phosphoric acid recoveredis a high purity feed grade phosphoric acid.

An additional feature of the present invention is that a greater amountof potassium ion as K₂ O may be present in the initial acidulation. Thisgreater amount should be sufficient to react with the phosphoric acidpresent and form potassium dihydrogen phosphate. In this aspect of thereaction the potassium is preferably present in the form of potassiumhydrogen sulfate (KHSO₄) or potassium sulfate (K₂ SO₄), or mixturesthereof, and is added in the stoichiometric amount necessary to combinewith the phosphoric acid produced as a result of the reaction.Therefore, at the separation stage the gypsum and potassiumsilicofluoride will be removed as described above but the resultingsolution will be a solution of potassium dihydrogen phosphate containedin phosphoric acid. This latter solution may be treated as describedhereinafter for recovery of potassium dihydrogen phosphate andphosphoric acid as desired.

The silica added during the reaction of this invention must be amorphoussilicon dioxide in any suitable form so long as it is not deleterious tothe reaction under consideration. The silica is preferably obtained frommaterials combinable with the phosphate rock, such as slag, orcommercially available products such as those sold under the tradename"Dicalite", sold by Grafco Corporation.

It has also been discovered in accordance with a further embodiment ofthis invention that a conventional wet process phosphoric acid facilitycan be conducted wherein the phosphate rock is acidulated with sulfuricacid under conditions as described above to produce a gypsum precipitateand phosphoric acid. In this system, a portion of the fluorides areevolved in the initial step and are trapped or recovered with aconventional scrubbing sytem. The precipitated gypsum is then removedfrom the phosphoric acid and the phosphoric acid is subjected to furthertreatments to effect defluorination of the phosphoric acid to produce ahigly pure feed grade acid. In these further treatments, the phosphoricacid is subjected to defluorination by the addition of silicon dioxidein any of the forms described herein together with the addition of K₂ Oin order to precipitate additional fluoride from the phosphoric acid aspotassium silicofluoride and yield feed grade H₃ PO₄. The defluorinationis effected by precipitation of any fluorides contained in the acid aspotassium silicofluoride which can then be removed from the phosphoricacid by means of a separator and treated for recovery of the fluoridesas calcium fluoride by the addition of calcium hydroxide so as toproduce potassium hydroxide and calcium fluoride.

The resulting pure phosphoric acid may then be recovered or optionally aportion thereof may be treated or reacted with phosphate rock, togetherwith potassium hydrogen sulfate or potassium sulfate, to produce amixture comprising precipitated gypsum, potassium dihydrogen phosphate,phosphoric acid and fluorides from the phosphate rock as potassiumsilicofluoride. This mixture may then be separated as desired to removethe precipitated gypsum and K₂ SiF₆, and recover a fertilizer gradeproduct comprising a mixture of phosphoric acid containing potassiumdihydrogen phosphate in solution which may then be recovered or treatedas desired such as by granulation. The mixture containing the remainderof the potassium dihydrogen phosphate and phosphoric acid may then berecycled to the defluorination step to provide the K₂ O values from thedefluorination stage. In an optional embodiment, all or a portion of thegypsum/K₂ SiF₆ mixture may be recycled to the defluorination stage toprovide both silicon dioxide and K₂ O values to the defluorinationstage.

Referring now to FIG. 1 accompanying the present invention, where a mainembodiment of the process is described, it will be seen that reactor orattack vessel 1 is provided and phosphate rock is introduced throughline 3 and sulfuric acid is introduced through line 2 to effect thereaction at 75° C., or other desired temperature within the rangespecified. The attack vessel is preferably maintained under conditionsof agitation, and K₂ O as potassium ion is introduced through line 4 asindicated. Simultaneously, silicon dioxide-containing salg is introducedthrough line 5. These components are reacted until substantiallycomplete acidulation is achieved and good gypsum crystal growth isachieved. During the reaction there is no detectable fluoride evolution.The resulting mixture is then transferred by line 6 to filter or otherseparator 7 where the solid and solution are separated with the solidbeing removed through line 8 to storage 9. The solid mixture containsthe gypsum or calcium sulfate in admixture with potassium silicofluorideand may be separated by extraction as described in our copendingapplication Ser. No. 696,289, filed June 15, 1976.

The solution from filter 7 comprising a solution of phosphoric acidcontaining the excess potassium ion as K₂ O is removed by line 10 toprecipitator or vessel 11 and to the solution in vessel 11 is addedadditional silicon dioxide through line 12 to effect furtherprecipitation of potassium silicofluoride. About 0.1 to 0.5% of silicondioxide based on the total amount of phosphoric acid product is added toeffect precipitation of the potassium silicofluoride. The resultingmixture is then removed by line 13 to filter 14 where the potassiumsilicofluoride is removed by line 16 and the resulting solution isrecovered at line 15 as highly pure feed grade phosphoric acid.

In an alternative embodiment of FIG. 1, if an excess of K₂ O is added toacidulation reactor 1, the resulting phosphoric acid from vessel 11,after removal of the potassium silicofluoride, will contain potassiumdihydrogen phosphate in the solution of phosphoric acid and would berecovered at line 15. The excess K₂ O in the main acidulation reactorwould be provided by the addition of KHSO₄, K₂ SO₄, or mixture throughline 4 in a stoichiometric amount to form the KH₂ PO₄. The potassiumdihydrogen phosphate could then be recovered from the phosphoric acid bymeans known to the art as by concentration of the phosphoric acidsolution to effect crystallization of the KH₂ PO₄ or by extraction ofthe phosphoric acid from the KH₂ PO₄ with an organic solvent. Processesfor separations of this type are described in U.S. Pat. Nos. 3,697,246and 3,718,453. In a further optional aspect, the solution of KH₂ PO₄ inH₃ PO₄ can be ammoniated by methods known in the art, granulated andmarketed as an N-P-K fertilizer.

In the continuous process of FIG. 2, there is set forth a complete andcomprehensive process by which the invention of FIG. 1 can be integratedinto an overall acidulation process for the production of phosphoricacid and for conversion of the phoshporic acid and other products touseful fertilizer products. Moreover, the continuous procedure of FIG. 2also provides means whereby the K₂ O and silica necessary to effect thereaction in the acidulation reactor can be provided as part of recyclestreams in order to improve the economics of the process.

Referring now more specifically to the process of FIG. 2, it will beseen that reactor 20 is provided as an acidulation reactor in which thephosphate rock is contacted with sulfuric acid in the presence of K₂ Oand silica as described for FIG 1. In main acidulation reactor 20,sulfuric acid is introduced through line 21 and phosphate rock throughline 22 with generally continuous agitation in reactor 20, the reactor20 being maintained at about 75° C. for a sufficient period to achievegood crystal growth. Water of dilution as necessary for line 23 is mixedwith the sulfuric acid in line 21 to maintain preferred concentrations.Also additional phosphoric acid from any desired source may beintroduced through line 23 to adjust or maintain the P₂ O₅ concentrationin the system within desired limits. The K₂ O and SiO₂ components aremaintained in the reactor 20 in the required amounts from recycle stream24 as described hereinafter.

After the acidulation reaction is complete in reactor 20, the mixture isremoved by line 25 to separator 26 which may be a filter or thickener,and gypsum and other precipitated materials are separated from thephosphoric acid solution and recovered in line 27. The solids recoveredfrom line 27 may be further treated to recover the K₂ SiF₆ from thegypsum by the methods described in our copending application referred toabove.

The phosphoric acid is removed from separator 26 by line 28 and can becompletely recovered at line 29, or a portion or all can be passed byline 30 to reactor 31 for reaction with phosphate rock. Preferably abouttwo-thirds by weight of H₃ PO₄ is removed from the system and one-thirdis passed to reactor 31.

In an optional but preferred embodiment, the phosphoric acid solutionfrom line 28, including any portion thereof, may be further purifiedrather than removing by line 29. In this embodiment the phosphoric acidis passed to precipitator 32, maintained at a temperature ranging fromabout 20° C. to 60° C. and an amorphous silicon dioxide material addedthereto with intimate mixing. The amount of silica material ranges fromabout 0.1 to about 0.5% of SiO₂ based on the total weight of phosphoricacid being treated. The resulting phosphoric acid containingprecipitated potassium silicofluoride is then removed by line 34 toseparator 35 and the K₂ SiF₆ is recovered at line 36. The resultingpurified feed grade phosphoric acid is removed and recovered at line 37.

In reactor 31, the phosphoric acid portion from line 30 is reacted withphosphate rock from line 38, possibly some sulfuric acid and eitherpotassium bisulfate (KHSO₄) or potassium sulfate from line 39. Reactor31 is maintained under conditions of agitation at a temperature of about50° to 90° C. In this reaction, the rock, possibly some sulfuric acid,KHSO₄ or K₂ SO₄, and phosphoric acid react to form KH₂ PO₄ and gypsumand thereby produce a solution of KH₂ PO₄ in phosphoric acid containingprecipitated gypsum.

The amounts of phosphate rock, sulfuric acid, and KHSO₄ and/or K₂ SO₄introduced into reactor 31 are added in such amounts as to product aresulting product mixture containing one mol of KH₂ PO₄ and one mol ofH₃ PO₄ calculated as P₂ O₅. This reaction slurry is passed by line 40 toseparation device 41 which is preferably a settler of known constructionfrom which an overflow and underflow can be recovered. From separator41, an overflow is continuously removed by line 42, the overflowsolution comprising a solution containing substantially equimolaramounts of KH₂ PO₄ and H₃ PO₄. The underflow from settler 41 is removedat line 43 and contains a mixture of KH₂ PO₄, KHSO₄ together withprecipitated gypsum and other solids including K₂ SiF₆. This mixture issuitable for recycle to main reactor 20 as it contains H₃ PO₄, K₂ O asKH₂ PO₄, and SiO₂. Thus the mixture is recycled from line 43 to line 24for introduction into reactor 20. In this embodiment, when utilizing oninitial reaction in reactor 20 of 15 moles phosphate rock and 150 molssulfuric acid, a recycle stream can be provided in line 25 containing 10mols KH₂ PO₄, 10 mols phosphoric acid, 30 mols of gypsum and 1 mol K₂SiF₆.

Therefore, by the system of FIG. 2, means are provided for effecting thereaction of phosphate rock and sulfuric acid with removal of thefluorides and silica as K₂ SiF₆ in the gypsum separated from thephosphoric acid and means are also provided for the formation of feedgrade phosphoric acid, a solution of KH₂ PO₄ in H₃ PO₄, and the recycleof necessary reactants to the acidulation reactors.

The system of FIG. 2 provides a number of advantages in operation of asystem of this type. Thus it provides a single separator 26 from whichgypsum solids can be recovered so that gypsum and potassiumsilicofluorides can be removed from the system in a single step and themixture further treated to recover the K₂ SiF₆ as described for examplein our copending application. Hence both the gypsum and K₂ SiF₆ arerecovered at a single separator. Further phosphoric acid is not requiredto be made in situ but is recycled from settler 41. Moreover, valuable Kand P containing products and pure phosphoric acid are recovered withoutfluorine pollution problems.

Referring now to FIG. 3 accompanying the present invention, whichillustrates the embodiment of the present invention wherein wet processphosphoric acid is prepared, it will be seen that a conventionalphosphoric acid reactor 50 is maintained for the reaction of phosphaterock introduced at 51 and sulfuric acid introduced at 52. The phosphoricacid reactor is maintained at a temperature of about 50°-90° C. withmeans provided at 53 for evolution of hydrogen fluoride as a result ofthe reaction. The hydrogen fluoride from 53 is trapped in a conventionalsystem of scrubbers and traps. During this reaction, the rock andsulfuric acid form a gypsum precipitate in phosphoric acid and thismixture is passed by line 54 to crystallizer 55 maintained at about20°-40° C. for complete precipitation of the gypsum. A portion of themixture from crystallizer 55 may be recycled through line 56 to flashcooler 57 and by line 58 to the main reactor in order to maintain thepreferred operating conditions in reactor 50.

The resulting slurry from crystallizer 55 is passed by line 59 to filteror other separator 60, preferably a Prayon filter, where a separation iseffected between the solid gypsum, which is removed by line 61, and thephosphoric acid filtrate. The filtrate is removed by line 62 todefluorination stage or vessel 63 for conducting the novel treatingsteps of this embodiment of the invention.

In defluorinator 63, which is maintained at a temperature of about20°-70° C., silicon dioxide containing materials are introduced throughline 64 and simultaneously, a source of K₂ O, preferably a mixture ofpotassium dihydrogen phosphate and phosphoric acid as part of a recyclestream, is introduced into defluorinator 63 by line 65 in order toprovide sufficient K₂ O to precipitate fluorides contained in thephosphoric acid as potassium silicofluoride. The SiO₂ and K₂ Ocontaining materials may also be any of those materials described abovewith respect to the main embodiment of FIGS. 1 and 2.

The resulting slurry from defluorination stage 63 is then passed by line66 to thickener 67 and then by line 68, at least a portion is passed toskimmer or equivalent device 69, which provides for withdrawal of theprecipitated potassium silicofluoride by line 70. The potassiumsilicofluoride in line 70 is passed to recovery tank 71 maintained at atemperature of about 20°-60° C. where it is further treated. Thusrecovery tank 71 is provided for the introduction of an aqueous solutionof calcium hydroxide from line 72 in sufficient amounts to precipitatethe fluorides present as calcium fluoride which will then free thesilica to precipitate as silicon dioxide which is removed by line 73.The resulting aqueous solution of potassium hydroxide and calciumfluoride in solution is removed through line 76 for recovery andseparation as desired.

The phosphoric acid solution from skimmer 69 is passed by line 75 tosurge tank 76 for recovery of a portion if desired. From thickener 67any overflow phosphoric acid is passed by line 77 directly to surge tank70. The desired amount of phosphoric acid, preferably about one-third,may be removed as feed grade phosphoric acid by line 78. The amounts ofphosphoric acid removed from the system may be varied as desired. Theremainder of the phosphoric acid is passed by line 79 to reactor 80maintained at a temperature of about 50°-90° C. where it is contactedwith phosphate rock from line 81 to form a slurry with some acidulationof the rock caused by the amount of phosphoric acid present. Theresulting slurry is then passed by line 82 to crystallizer 83 maintainedat a temperature of about 40°-60° C. into which sufficient potassiumhydrogen sulfate or potassium sulfate is introduced by line 84 toachieve further reaction in crystallizer 83 and form a resulting mixtureof KH₂ PO₄ in phosphoric acid solution, together with precipitatedgypsum which may also contain solid potassium silicofluoride formed byreaction of potassium with the fluoride in the rock. This mixture isthen passed by line 85 to gypsum filter 86 wherein the gypsum andpotassium silicofluoride are removed by line 87. The resulting solutionis passed by line 88 to storage tank 89 from which a portion of themother liquor product is removed through line 90. This product comprisesa mixture of potassium dihydrogen phosphate in phosphoric acid solutioncontaining high analysis values of P₂ O₅ /K₂ O, separated from thegypsum and K₂ SiF₆. A portion of for example up to about one-third ofthe mixture of KH₂ PO₄ in phosphoric acid, is recycled by line 91 toline 65 for introduction into the defluorination stage 62 to provide theK₂ O values for the defluorination step.

Thus by the reaction scheme of FIG. 3, means are provided for conductinga conventional phosphoric acid facility wherein the phosphoric acidproduct can be further treated to achieve defluorination and for theproduction of a fertilizer material comprising substantially equalamounts of phosphoric acid and KH₂ PO₄. Further, means are provided forrecovery of the fluorides as usable calcium fluoride.

In an alternative embodiment on the schematic of FIG. 3, the reactionslurry from crystallizer 83 may be passed to a thickener or similarseparator rather than a gypsum filter and an overflow removed which willcontain a solution of KH₂ PO₄ in phosphoric acid to provide the P₂ O₅/K₂ O product desired. The resulting underflow, which will contain KH₂PO₄, phosphoric acid, precipitated gypsum and K₂ SiF₆, is then recycledto the defluorination step and will provide K₂ O values as well as SiO₂values, for the defluorination stage.

The following examples are presented to illustrate the invention but itis not to be considered as limited thereto. In the examples andthroughout the specification, parts are by weight unless otherwiseindicated.

EXAMPLE 1

To demonstrate the embodiment of FIG. 1 and show that the fluorides canbe removed from phosphoric acid by the addition of K₂ O and SiO₂, thefollowing experiments were carried out.

It was discovered that to obtain feed grade phosphoric acid, the weightratio to P₂ O₅ /F must be at least 230. Therefore, in the followingexperiments KH₂ PO₄ was added to wet phosphoric acid obtained from Condarock to provide about 1.5 to 2.0 weight percent K₂ O in the acid afterthe K₂ SiF₆ had precipitated. With a minimum of about 0.7 weight percentsoluble K₂ O in the acid the fluorine concentration was about 0.25 to0.30. Increasing the K₂ O level as high as 4 to 6 weight percent did notsignificantly reduce the fluorine level. In addition to the KH₂ PO₄,different silica bearing materials were added at differentconcentrations. The samples are equilibrated at 40° C. for about 16hours and filtered at 40° C., and then the filtrates were analyzed.

The results tabulated in Table I show that with K₂ O addition only, thefluorine concentration was about 0.25 weight percent. The followingsilica materials reduced the fluorine level to 0.15-0.18 weight percent:Dicalites 425, BA-3, BA-30, SA-3, DPS-20, SA-33, Superaid, and slag fromDuval's Ferro-Moly plant. These several "Dicalite"0 products comprise aseries of silica-containing products sold under that trademark by GrafcoCorporation. The slag was obtained from the Ferro-Moly plant of DuvalCorporation. Even at an F concentration of 0.15-0.18 the P₂ O₅ /F weightratio was only about 175 which is not high enough for feed grade acid.This F concentration corresponds closely to the K₂ SiF₆ solubility in H₃PO₄ at 20-30% P₂ O₅.

According to the K₂ SiF₆ solubility in H₃ PO₄ curves, the P₂ O₅ /Fweight ratio can be increased by concentrating the acid to 50-60% P₂ O₅.A number of tests were run during which different samples wereconcentrated to 50-60% P₂ O₅. It was found that there could be obtainedP₂ O₅ /F weight ratios higher than 230 by adding KH₂ PO₄ and additionalsilica bearing materials to wet phosphoric acid and then concentrating.

TABLE I ADDITION OF KH₂ PO₄ AND SILICA-BEARING MATERIAL TO WET ACID

Samples equilibrated at 40° C. for about 16 hours. For each sample 93.5gms of wet acid and 6.5 gms of KH₂ PO₄ were used. Composition of WetAcid: K₂ O-0.04%; SiO₂ -0.99%; F-1.11%; P₂ O₅ -28.5%.

    ______________________________________                                                          Gms      Resulting                                          Ex.  Silica Material                                                                            Material Filtrate Analysis (Wt. %)                          No.  Added        Added    K.sub.2 O                                                                          SiO.sub.2                                                                          F    P.sub.2 O.sub.5 /F                  ______________________________________                                        1    .sup.(1) Dicalite 425                                                                      0.0      1.77 0.0  0.25 123                                 2    Dicalite 425 0.1      1.68 0.02 0.20 148                                 3    Dicalite 425 0.5      1.46 0.02 0.19 150                                 4    Dicalite 425 1.0      1.80 0.02 0.18 170                                 5    Dicalite                                                                      Superaid     0.0      1.71 0.0 0.24                                                                           125                                      6    Dicalite                                                                      Superaid     0.1      1.76 0.0  0.24 125                                 7    Dicalite                                                                      Superaid     0.5      1.76 0.0  0.18 165                                 8    Dicalite                                                                      Superaid     1.0      2.02 0.0  0.15 226                                 9    Dicalite BA-3                                                                              0.1      1.69 0.02 0.16 185                                 10   Dicalite BA-3                                                                              0.5      1.70 0.02 0.17 168                                 11   Dicalite BA-3                                                                              1.0      1.82 0.02 0.16 190                                 12   Dicalite BA-30                                                                             1.0      1.74 0.02 0.15 200                                 13   Dicalite SA-3                                                                              1.0      2.01 0.02 0.17 177                                 14   Dicalite DPS-20                                                                            1.0      1.74 0.02 0.17 177                                 15   Dicalite SA-33                                                                             1.0      1.81 0.02 0.18 167                                 16   .sup.(2) Duval Slag                                                                        0.0      1.73 0.02 0.20 152                                 17   Duval Slag   0.1      1.75 0.02 0.17 173                                 18   Duval Slag   0.5      1.71 0.02 0.17 170                                 19   Duval Slag   1.0      1.65 0.02 0.17 173                                 ______________________________________                                         .sup.(1) Dicalite - silica product sold by Grafco Corporation                 .sup.(2) Duval Slag - obtained from Duval Corporation                    

EXAMPLE 2

In this example the reaction was conducted according to the flow sheetof FIG. 2.

Into the main reactor was introduced 15 mols of phosphate rock and 150mols of sulfuric acid and the acidulation reaction was conducted a 75°C. with agitation. A recycle stream provided 10 mols KH₂ PO₄, 10 molsphosphoric acid, 20 mols gypsum and 1 mol K₂ SiF₆ and a second streamprovided phosphoric acid containing 43 mols P₂ O₅. Phosphoric acid wasobtained from the first settler and two-thirds was recovered. The otherone-third of the phosphoric acid was reacted at stoichiometric ratioswith phosphate rock and KHSO₄ at a 30% solids concentration to producepotassium dihydrogen phosphate and one-third of the KH₂ PO₄ in H₃ PO₄solution was recovered from the settler. The other two-thirds of the KH₂PO₄ together with the H₃ PO₄, gypsum and other precipitated material,were recycled to provide the recycle stream to the main acidulationreactor.

EXAMPLE 3

In this example the reaction was conducted according to the embodimentof FIG. 3.

Into the main reactor was introduced phosphate rock at a rate of 45mols/hour of P₂ O₅ for reaction with sulfuric acid introduced at a rateof 150 mols/hour for reaction at 75° C. The reactor was provided withagitation and the temperature was maintained by recycle of a portion ofthe reaction mixture through a flash cooler. Evolved fluorides weretrapped in a conventional water scrubber system. The resulting slurrywas passed to a Prayon filter from which 150 mols/hour of calciumsulfate were removed in the continuous procedure. The resultingphosphoric acid solution was then passed to a defluorination stage whereit was reacted at a temperature of about 60° C. with silicon dioxide anda recycle stream providing 10 mols of P₂ O₅ and 10 mols of K₂ O.Sufficient SiO₂ was introduced as Dicalite to react with the fluorinepresent in the H₃ PO₄ as determined by analysis of the H₃ PO₄. A portionof the reacted mixture containing precipitated fluorides as K₂ SiF₆, isthen passed to a thickener and underflow from the thickener is passed toa skimmer from which 5 mols of K₂ SiF₆ are removed as a precipitate fromthe system. The K₂ SiF₆ is then reacted with 15 mols calcium hydroxideat 60° C. Under these conditions, a silicon dioxide precipitate formsand is removed from the system to provide a solution containing 10 molsKOH and 15 mols CaF₂.

The resulting solution from the skimmer of phosphoric acid, togetherwith overflow H₃ PO₄ from the thickener, are passed to a surge tank fromwhich a phosphoric acid solution containing 36 mols P₂ O₅ and a N-P-Kvalue of 0-30-0.3 is removed from the system as feed grade phosphoricacid.

The remaining H₃ PO₄ from the surge tank, containing 19 mols P₂ O₅ ispassed to a reactor maintained at 75° C. and reacted with phosphate rockcontaining 9 mols P₂ O₅. The resulting slurry is then passed to acrystallizer maintained at 50° C. into which is introduced 30 mols KHSO₄containing 15 mols K₂ O. The resulting slurry is then filtered to removeand recover a gypsum solids mixtures containing 30 mols of gypsum and 1mol of K₂ SiF₆. The resulting filtrate solution, which now has an N-P-Kvalue of 0-24-8, is then separated and a mixture removed from the systemwhich contains 18 mols of P₂ O₅ and 9 mols K₂ O and contains thesematerials substantially as KH₂ PO₄ and H₃ PO₄. This product may begranulated for use as a fertilizer or otherwise treated as desired. Themixture not removed from the system is recycled to the defluorinationstep for reaction with phosphoric acid and silicon dioxide to provide 10mols P₂ O₅ recycle.

EXAMPLE 4

Example 3 was repeated except that the phosphoric acid solution, afterreaction with phosphate rock and KHSO₄ in the crystallizer, was passedto a thickener where an overflow was continuously removed containing 18mols of P₂ O₅ and 9 mols K₂ O and this mixture, having an N-P-K value of0-24-8 is removed from the system for further treatment as desired suchas granulation. The resulting underflow from the thickener containing 10mols KH₂ PO₄, 10 mols phosphoric acid, 30 mols gypsum and 1 molpotassium silicofluoride, is recycled to the defluorination stage toprovide K₂ O and SiO₂ values to the defluorination.

The invention has been described herein with reference to certainpreferred embodiments. However, as obvious variations thereon willbecome apparent to those skilled in the art, the invention is not to beconsidered as limited thereto.

What is claimed is:
 1. A method for the preparation of phosphoric acidby the steps which comprise:a. reacting a fluorine-containing phosphaterock with sufficient sulfuric acid to acidulate said rock in anacidulation reactor, said reaction being conducted in the presence of:1.potassium ion provided by addition of a member selected from the groupconsisting of KH₂ PO₄, KHSO₄, K₂ SO₄, KOH and mixtures thereof insufficient amounts to combine with the fluorine liberated from thephosphate rock during acidulation and in a sufficient excess amount thatthe phosphoric acid recovered from the reaction will contain about 0.5to 1.0 weight percent potassium ion as K₂ O; and
 2. in the presence of asufficient amount of silicon dioxide to combine with the fluorineliberated during acidulation of the rock and a sufficient excess amountof the silicon dioxide to provide about 0.003 to 0.015 parts of silicondioxide per part of P₂ O₅ in the phosphoric acid recovered; b. forming aresulting slurry of gypsum solids in phosphoric acid and removing theslurry from the acidulation reactor; and c. separating the gypsum solidsfrom the phosphoric acid and recovering gypsum solids containingprecipitated potassium silicofluoride and a phosphoric acid solutionsubstantially free of fluorine.
 2. A method according to claim 1 whereinthe acidulation reaction is conducted at a temperature of about 50° to90° C.
 3. A method according to claim 2 wherein the potassium ion isprovided by the addition of KH₂ PO₄.
 4. A method according to claim 1wherein the reaction is conducted at a temperature of 60° to 75° C.
 5. Amethod according to claim 1 wherein the silicon dioxide is provided bythe addition of an amorphous silicon dioxide.
 6. A method according toclaim 1 wherein the recovered phosphoric acid solution product isfurther contacted with 0.1 to 0.5% of silicon dioxide based on the totalweight of phosphoric acid solution to precipitate additional K₂ SiF₆from the phosphoric acid, removing the K₂ SiF₆, and recovering a feedgrade phosphoric acid.
 7. A method according to claim 1 wherein asufficient amount of potassium ion is present in the acidulation reactorto form KH₂ PO₄ by reaction with a portion of the phosphoric acid andthe phosphoric acid product recovered comprises a solution of KH₂ PO₄ inphosphoric acid.